1. Field of the Invention
The present invention relates to the field of sealing of sliding interfaces. More particularly, the present invention relates to the field of sealing of sliding interfaces, such as that existing between a cylinder and a piston reciprocating therein, or between a bushing and a rod reciprocating within an aperture through the bushing, where a difference in pressure exists between or across opposed faces or surfaces of the moving member.
2. Background of the Art
Sliding seal interfaces are used to enable the transfer of energy, such as from combustion or from temperature or pressure changes of a gas or fluid in an enclosed volume, by enabling a member such as a piston to move relative to a cylinder, and thus expand or contract the volume enclosed by the cylinder and piston surfaces in response to such combustion or pressure or temperature changes, and thereby enable the energy, evidenced for example by an increase in fluid pressure in a volume bounded by a piston and a cylinder, to be reduced in that volume by increasing the volume within which the fluid is present, resulting in useful movement of the piston to transfer energy to a useful output, such as a shaft used to drive an electrical generator, a wheel, or other mechanical or electromechanical device.
There exist several fundamental issues with the recovery of energy from a reciprocating piston driven by pressure-volume energy changes in a cylinder. To maximize the energy recovered by a change in pressure within the volume bounded by the cylinder and piston surfaces, leakage of the fluid under pressure past the interface between the piston outer surface and the adjacent cylinder inner surface must be minimized, which dictates a tight seal, yet friction (and wear) caused by physical contact of the piston with the cylinder must also be minimized, which dictates little or no contact between moving parts. The same paradigm is present where the converse situation is present, where energy is being transferred from a mechanical apparatus into a fluid, such as where a fluid is being compressed by reducing its volume within a piston-cylinder system. In either situation, loss of fluid under pressure through the interface, as well as friction at the interface, reduces the energy recovered from or put into the fluid, and hence the efficiency of the device.
One known mechanism for sealing a piston-cylinder interface is to employ a split ring, also known as a Ramsbottom seal, in a groove in the piston wall such that the seal moves with the piston and seals across the piston-cylinder interface. The seal is commonly, for example, a square cut split metal ring, which is received within a mating square cut groove in the outer cylindrical surface of the piston, and the free diameter of the ring or seal is larger than the inner diameter of the cylinder, which ensures expansion or bias of the outer surface of the ring toward the inner circumferential wall of the cylinder. Oil or other lubricant is commonly, but not always, used to lubricate the contact area between the seal and the inner wall of the cylinder, which provides a mechanism to remove heat from the sliding interface and reduce friction where the outer face of the ring rides on a thin film of oil and not directly in contact with the cylinder wall. However, in applications where lubricants cannot or should not be used, for example, where a seal must be maintained across alternating low pressure and high pressure sides of a piston located in a cylinder used in a Stirling engine, the lack of lubrication contraindicates the use of a Ramsbottom or other contact-type seal, because of the high contact friction between the seal and cylinder wall, which leads to energy losses, and the high wear of the ring and/or the cylinder wall, which will require frequent replacement of one or both. These issues have limited the application of a simple Ramsbottom seal arrangement in such no- or low-lubricant applications.
One mechanism which has been used to provide a non-lubricated sliding seal is a clearance seal between the outer circumferential surface of the piston and inner circumferential surface of the cylinder within which the piston reciprocates. A clearance seal is formed by an intended, minute radial gap between the outer circumferential wall of the piston and adjacent interior cylinder wall surfaces. In theory, the piston may reciprocate in the cylinder on a hydrodynamic gas bearing formed by the very thin cushion of a gas or fluid which may be created between the outer circumferential wall of the piston and the inner circumferential wall of the cylinder by their relative motion. In practice, however, such a bearing is formed by supplying the gap with pressurized gas, making the bearing a hydrostatic bearing, requiring a supply of this pressurized gas. If it is desired that the piston not contact the cylinder, but not incur the expense of the hydrostatic gas bearing and associated components to provide pressurization, exacting alignment of the piston with respect to the cylinder is required of the mechanism that provides or receives mechanical energy to or from the piston respectively, to cantilever the piston off of this mechanism into the cylinder so that the piston moves within the cylinder without touching the cylinder wall. Because the clearance seal provides a leakage path between a high and low pressure side of the piston, an inherent energy and efficiency loss is present, exacerbated by the energy consumed to pressurize a hydrostatic gas bearing if used, but is tolerated as an acceptable trade off to enable non-lubricated sealing of the piston-cylinder interface.